1. Field of the Invention
This invention relates to introducing alumina into an aluminum reduction cell, and more particularly, to methods and apparatus for feeding alumina to a Soderberg-type aluminum reduction cell.
The production of aluminum by electrolysis of an aluminum oxide containing material is a well-known process. Commercial production of aluminum is carried out in a reduction cell by the Hall-Heroult process in which alumina is dissolved in molten cryolite (NaF/NaAlF4) at about 970.degree. C. The aluminum ion is then electrolytically reduced by an electrical current which is passed through the electrolyte. The resulting purified aluminum is collected as a molten metal in a pool beneath the molten electrolyte bath, and periodically removed by a vacuum tap.
Electrical current enters the cell through an anode in contact with the upper surface of the electrolyte bath, passes downward through the electrolyte bath, through the pool of molten aluminum and into a cathode which is integral with the bottom portion of the cell. The current leaves the cell through the cathode and is conducted to the anode of the next in a series of cells which make up a pot line.
The operating temperature of the bath is maintained by resistance heating of the bath, reactions in the cell, and insulating the cell structure.
One well-known reduction cell for carrying out this process is the self-baking anode cell, known as the Soderberg cell. In the Soderberg cell, the anode is continuously formed in place over the rectangular molten electrolyte bath by the baking action of heat from the bath on a carbon paste contained within a hollow steel jacket. Attached to the bottom edge of the anode jacket is a gas collection skirt to contain and exhaust gases produced in the process. As the lower "working surface" of the anode is consumed, the anode is lowered to maintain the desired anode-to-cathode distance, and more carbon paste is added to the top of the anode. As the carbon paste travels slowly downward through the jacket toward the molten bath, it is consolidated by heat and pressure into a compacted anode.
The anode is centered over the molten electrolyte bath for optimal current distribution through the bath. At each end of the cell are electrical connections between the cell and the electrical current distribution system. Along each side edge of the cell, a peripheral region of the molten electrolyte bath extends beyond the anode. Alumina feed material is fed into this peripheral region of the molten electrolyte bath.
Cooling of the surface of the exposed peripheral region of the molten electrolyte bath causes a frozen crust to form which extends from the gas collection skirt to the edge of the bath. If undisturbed, this peripheral region of the bath will freeze solid. Since it is through this region of the bath that alumina is fed to the cell, the electrolyte in this region must remain molten to receive the feed. This is achieved by periodically breaking through the solid crust with a mobile crust breaker, or a powered crust breaker apparatus fitted to the cell structure. The broken crust collapses into the molten pool and remelts leaving an opening in the crust. Alumina is then fed through the opening into the molten bath, and a solid frozen crust subsequently reforms over the edge of the bath.
Economical production of aluminum by electrolytic reduction requires efficient utilization of electrical energy. Numerous factors affect the electrical efficiency of a reduction cell, including current distribution in the molten electrolyte bath, resistance losses in the bath and associated electrical equipment, anode-to-cathode distance within the cell, the amount of aluminum oxide dissolved in the molten electrolyte, the presence of undissolved alumina in the cell, heat losses from the cell, electrolyte temperature, and others. Previous efforts to improve the electrical efficiency of the Soderberg cell have addressed these areas.
To optimize current distribution in the molten electrolyte, and to minimize the electrical resistance of the anode, the width of the anode relative to the molten bath was maximized. With the increased anode width, however, the width of the exposed peripheral region was reduced to the minimum required to allow feeding of alumina to the bath.
In order to minimize the electrical resistance losses in the molten electrolyte, the bath depth was reduced, thereby reducing the anode-to-cathode distance. In a modern Soderberg cell, the electrolyte bath depth is the minimum required for electrolyte bath stability. In order to maintain the desired bath temperature in light of attendant reduced resistance heating of the bath, insulation was added to the bottom of the cell.
The electrical resistance of the electrolyte bath is also affected by the amount of dissolved alumina in the bath. A deficiency of dissolved alumina increases the electrical resistance of the electrolyte, and decreases cell efficiency. The onset of this so called "anode effect" signals cell operators to add a batch of alumina feed to the cell.
The amount of alumina which can be fed to the cell in one batch is limited to the amount that will readily dissolve in the molten electrolyte bath. Excess alumina fed to the cell remains undissolved and may settle on the cathode, increasing the resistance of the cell and reducing the cell's efficiency (sick cell condition).
The operation of a Soderberg cell can therefore be characterized as a series of cycles of alumina saturation and depletion in the molten electrolyte, accompanied by attendant cycles of increasing and decreasing cell efficiency. Efforts to level these cycles by maintaining a more constant alumina concentration in the electrolyte have not been successful with the Soderberg cell. The electrical efficiency of a modern Soderberg cell, with the foregoing features and limitations, typically ranges from 85-88%.
The potential for a competitive advantage from increased electrical efficiency, and the limitations of the Soderberg cell, have led to an improved reduction cell design known as the pre-baked anode cell. The pre-baked anode cell design allows feeding of alumina into the center portion of the bath. Two advantages flow from center alumina feed. First, alumina fed to the center of the bath dissolves and disperses throughout the working area of the bath more quickly, minimizing the potential for undissolved alumina to accumulate in the cell. Secondly, there is no frozen crust in the center of the bath which must be broken prior to feeding. This feature allows alumina to be fed frequently in small amounts, referred to as continuous feeding, without disrupting the heat balance of the cell. By providing a more constant dissolved alumina concentration in the molten electrolyte, the alumina saturation and depletion cycles inherent in the Soderberg cell design are largely eliminated. The electrical efficiency of the pre-baked anode cell is increased to 92-96%. This represents a significant efficiency gain, and provides a competitive advantage for aluminum producers who employ pre-baked anode reduction cells.
In light of the competitive nature of the primary aluminum industry, continually increasing costs of electrical energy, and the significant efficiency advantage of the pre-baked anode cell, aluminum producers using Soderberg cells are forced to choose between an enormous capital investment to replace their Soderberg cells with pre-baked anode cells, or continued operation with their existing cells at a competitive disadvantage. What is needed, therefore, is a way to improve the efficiency of existing Soderberg cells in order to make them more competitive with pre-baked anode cells.
2. Description of Related Art
U.S. Pat. No. 4,016,053 to Stankovich et al discloses a feed distribution system for use in aluminum production plants. The '053 invention distributes alumina from a central storage bin to the vicinity of reduction cells (pre-baked or Soderberg type) throughout a plant by means of air fluidized conveyors. The '053 patent does not disclose a way to improve the efficiency of a Soderberg cell.
U.S. Pat. No. 3,888,747 to Murphy discloses a system for sensing the onset of an anode effect in a reduction cell by detecting an increase in the cell voltage, and for responding by producing an output signal. The system may include devices for detecting the output signal and feeding alumina to the bath in response to the signal. The '747 patent teaches routinely feeding alumina to the cell in an amount less than that required to prevent the onset of anode effects, then, at the onset of an anode effect, feeding an additional amount of alumina to the cell. In each instance, alumina is fed to the cell by storing a batch of alumina on top of the solid crust covering the edge of the bath, then breaking the solid crust and forcing the crust and alumina into the bath.
U.S. Pat. No. 4,431,491 to Bonney et al teaches a process and apparatus for correlating measured cell resistance values with the required amount of alumina feed for controlling the dissolved alumina content in the electrolyte, and for introducing the required amount of feed into the bath through a continuously open portion of the bath. The peripheral portion of the bath in a modern Soderberg cell cannot be kept continuously open, however, because unacceptable heat loss and operating conditions would ensue.
U.S. Pat. No. 4.035,251 to Shiver et al discloses a method of operating an alumina reduction cell by monitoring the cell resistance changes, and regulating the addition of alumina to the cell responsive to the cell resistance changes. As applied to the Soderberg cell, the method is used to trigger a typical batch feeding step where alumina is fed into the peripheral portion of the bath after breaking the solid crust.
It is against this background that the present invention was developed.